The present invention relates to a power supply circuit for providing a liquid crystal display with desired voltage levels. More particularly, the present invention relates to the power supply circuit of the above described type which can simplify circuit configuration and then facilitate fabrication of the power supply circuit.
The inventors have proposed an earlier power supply circuit suitable for supplying a liquid crystal energizing circuit with desired voltage levels or potentials as shown and described in copending application, FOUR-LEVEL VOLTAGE SUPPLY FOR LIQUID CRYSTAL DISPLAY, Ser. No. 685,261, now U.S. Pat. No. 4,050,064, filed May 11, 1976 by Shintaro Hashimoto and Yuuichi Sato and assigned to the same assignee as the present invention, the disclosure of which is incorporated herein by reference. The earlier circuit as shown in FIG. 1, includes an input terminal In.sub.1 connected to a reference potential or OV, another input terminal In.sub.2 connected to a constant voltage source VC and output terminals a-e for providing the liquid crystal display energizing circuit with desired potentials. The input terminal In.sub.1 is connected directly to the output terminal a and the second input terminal In.sub.2 is connected directly to the output terminal e. Resistors R.sub.1, R.sub.2, R.sub.3 and R.sub.4 of the substantially same resistance value are serially connected between the input terminals In.sub.1 and In.sub.2 and the output terminals b, c and d are coupled with the respective middle points of the series circuit of R.sub.1, R.sub.2, R.sub.3 and R.sub.4.
There is also provided a complementary MOS circuit which comprises a P channel MOS transistor Tr.sub.8 connected in parallel with the resistor R.sub.1 and an N channel MOS transistor Tr.sub.9 connected in parallel with the resistor R.sub.4. The transistors Tr.sub.8 and Tr.sub.9 are switchable between ON and OFF states in response to control signals A.
When the control signal A is OV, the transistor Tr.sub.9 is ON to establish a short circuit between In.sub.2 and d such that voltage between In.sub.1 and In.sub.2 is divided through the use of the resistors R.sub.1, R.sub.2 and R.sub.3 to produce outputs VA, VB and VC via terminals b-d. It is concluded that a = OV, b = VA, c = VB and d = e = VC. For the DSM type of liquid crystal displays with an ignition voltage of 18V, VC = -18V, VB = -12V and VA = -6V.
Conversely, when the control signal A is VC (-18V), the transistor Tr.sub.8 is ON to establish a short circuit between In.sub.1 and b such that voltage between In.sub.1 and In.sub.2 is divided by R.sub.2, R.sub.3 and R.sub.4 thereby to produce outputs VA and VB via C and D at terminals c, d, and e. As a consequence, a = b = OV, c = VA, d = VB and e = VC.
Nevertheless, although the above described power supply circuit including as the switching means the complementary MOS circuit as shown in FIG. 1 is advantageous from the viewpoint of circuit technique since the complementary MOS circuit provides output voltage levels approximately equal to the input voltage levels, it is still difficult to fabricate the complementary MOS circuit configuration at a low cost.
Accordingly, it is an object of the present invention to provide an improvement in the earlier proposed liquid crystal display driving circuit which does not include a complementary MOS transistor circuit configuration as a switching means. In accordance with the concept of the present invention, the power supply circuit for the liquid crystal display energizing circuit is adapted in such a way as to constitute the switching means by only P channel MOS transistors (or N channel MOS transistors).